The need for optical isolators in laser systems is well known. The basic purpose of an optical isolator is to only allow the passage of light therethrough in one (forward) direction by efficiently blocking light transmission in the reverse direction. Optical isolators are typically used to prevent unwanted feedback (i.e. backscattered light) into an optical oscillator, such as a laser cavity, where it can cause instability or even failure of the laser.
The quality (i.e.) performance of an optical isolator is typically measured in terms of several parameters, most notably (1) the insertion loss and (2) the isolation ratio. The insertion loss is a measure of the additional loss (i.e. attenuation) imparted to the outgoing laser beam due to the addition of the optical isolator in the beam path. The isolation ratio is the loss (i.e. attenuation) deliberately imparted to returning light trying to get through the isolator the “wrong way”. i.e. heading back towards the laser. The isolation ratio is ideally as high (in absolute value) as possible, but in practice varies considerably with values from −25 to −40 dB for operation at around 20° C. commonly encountered in commercially available units.
The isolation performance of conventional optical isolators is generally sensitive to both operating wavelength as well as temperature. Typically the wavelength of an industrial laser is very accurately known and stable, for example for a YAG laser with a nominal wavelength of 1064 nm, so that wavelength variation is generally not a significant problem. The effect of temperature on optical isolation, however, is generally far more serious.
A conventional optical isolator is a two port device that comprises in an optical path an input polarizer, a 45° Faraday rotator and a 45° output polarizer. In operation, linearly polarized light from a light source such as a laser (e.g. vertically polarized light) passes straight through the input polarizer, gets rotated +45° by the Faraday rotator, then passes straight through the 45°-orientated output polarizer (also referred to as an analyzer) with (ideally) no insertion loss. If a mirror is placed after the output polarizer, light will be reflected back and pass through the output polarizer in the opposite direction (i.e. the reverse direction), will then get rotated by another 45° by the Faraday rotator, so it becomes horizontally polarized when it encounters the input polarizer. Thus, the returning horizontally polarized light is blocked from reaching the light source (e.g. laser) by the input polarizer.
However, if the returning light is partially depolarized by the mirror, the returning light is still largely blocked, however, some of the returning light is blocked at the output polarizer and the remainder is blocked at the input polarizer. The key to this process is the Faraday rotator, also called a non-reciprocal rotator, which has the property of rotating the polarization of incident light in the same direction irrespective of the direction of light propagation. Such a device is well-known and is typically constructed by using a Terbium-doped glass rod or a terbium gallium garnet (TGG) crystal (in the form of a rod) placed in an axial magnetic field. The axial magnetic field is conventionally achieved using a magnet in the form of a tube, with the direction of magnetization along the axis of the tube, with the TGG rod placed in the central hole. The magnitude of the polarization rotation provided by the Faraday rotator varies with temperature, which generally decreases in magnitude with increasing temperature.
For example, a Faraday rotator designed to operate at 20° C. to produce a 45° rotation will typically produce progressively less rotation as the temperature is increased above 20° C. The effect of variation of temperature on optical isolation can be a very serious problem so that, the isolation in a physically realizable device may in practice be −70 dB at a design temperature, of 20° C., falling to only about −23 dB at both 0° C. and 40° C.